Communications satellites which receive analogue signals, for example from a ground-based transmitter or from another satellite, can be arranged to convert the analogue signals into the digital domain for onboard processing. The received analogue signal is converted using an analogue-to-digital converter (ADC), which measures the signal level, e.g. voltage or current, at a particular time and outputs a codeword representing the measured signal level. Therefore the digital signal output by the ADC comprises a sequence of codewords representing the variation in the received signal level over time.
Also, in signal processing applications it can be necessary to accurately measure the phase angle of the signal being processed. For example, when quadrature amplitude modulation (QAM) schemes are used, the angular separation between distinct symbols referred to the origin of the constellation decreases as the order of modulation is increased. Therefore accurate phase angle measurement is desirable to ensure that symbols can be reliably distinguished. Errors can be present in the signal as-received, for example as Gaussian noise, or can be introduced by various components before the digital signal processor (DSP). In the case of conversion from an analogue to a digital signal by an ADC, systematic errors can be introduced by the ADC itself. For instance, it is known that ADCs can suffer from integral non-linearity (INL) and differential non-linearity (DNL) effects which reduce the dynamic range for which the ADC can be used.
The invention is made in this context.